I hav a green tweed jacket and was wandering what kind of trousers to wear with it if im styling it for a casual/smart occasion, also if i should wear a shirt underneath or try go for a t-shirt to make it look more casual?

First thing's first: t-shirt under a blazer is a look which was briefly fashionable for a few years, but has dropped out of fashion. That's a good thing, because the contrast between the formal lines of a blazer and the informality of a collarless t-shirt made it an awkward look - so yes, wear a shirt.

But: you don't have to have a very formal shirt, nor do you have to wear a tie with it. An oxford button-down shirt might look good; but if you think a thick cotton shirt with a thick tweed jacket is a bit too warm, maybe a summery linen shirt in a plain colour?

As for trousers, for a casual/smart occasion you want to hit that sweet spot between too smart and too casual:

Don't wear dark trousers from a suit; the smartness of the trousers (the clean lines, the smooth fabric, the plain colour) won't match the rest of your outfit.

A pair of chinos might work well; the coarser fabric makes them less formal than suit trousers.

Or, grey flannel trousers - ditto.

Going from the casual end, a pair of loose-fit jeans with rips and faded patches would be too casual.

The kind of jeans which might work well would have to be consistently coloured (grey, indigo, black) and with clean lines: straight cut, tapered or slim (but not skinny).

If you have a slim/skinny fit tweed blazer then yeah why not? IMO skinny jeans when done right can look very smart, especially compared to even a lot of the tapered and especially the straight cuts you see, and have an edginess and youth to them at the same time. There's a big difference between the tapered jeans you buy in the shops and the ones, say, Creed wears where he's had a tailor/seamstress taper them for him to get the perfect smart/casual fit.

Don't see the problem with a t-shirt and skinnies with a blazer. It just depends on the blazer. If it's casual (e.g. unstructured and textured), then that leaves loads of scope for what you can pair with it. Just don't do it with a suit jacket.

skinnies are just about as versatile as it gets in my opinion and they show off your shoes in a better way than other jean cuts. For tee with blazer look the blazer has to be deconstructed enough to work in the more casual sense. You see too many blokes wearing a tee paired with their badly fitted suit blazer from work attempting to look trendy but missing the mark all the more for it

Thanks for the info guys il have to try a few things together and see what i think looks better, id say that the colour is a more green/brown and i dont really suit skinys but il try a thinner pair of trousers.